Designation C1470 − 06 (Reapproved 2013) Standard Guide for Testing the Thermal Properties of Advanced Ceramics1 This standard is issued under the fixed designation C1470; the number immediately follo[.]
Trang 1Designation: C1470−06 (Reapproved 2013)
Standard Guide for
This standard is issued under the fixed designation C1470; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision A number in parentheses indicates the year of last reapproval A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1 Scope
1.1 This guide covers the thermal property testing of
ad-vanced ceramics, to include monolithic ceramics, particulate/
whisker-reinforced ceramics, and continuous fiber-reinforced
ceramic composites It is intended to provide guidance and
information to users on the special considerations involved in
determining the thermal properties of these ceramic materials
1.2 Five thermal properties (specific heat capacity, thermal
conductivity, thermal diffusivity, thermal expansion, and
emittance/emissivity) are presented in terms of their definitions
and general test methods The relationship between thermal
properties and the composition, microstructure, and processing
of advanced ceramics (monolithic and composite) is briefly
outlined, providing guidance on which material and specimen
characteristics have to be considered in evaluating the thermal
properties of advanced ceramics Additional sections describe
sampling considerations, test specimen preparation, and
report-ing requirements
1.3 Current ASTM test methods for thermal properties are
tabulated in terms of test method concept, testing range,
specimen requirements, standards/reference materials,
capabilities, limitations, precision, and special instructions for
monolithic and composite ceramics
1.4 This guide is based on the use of current ASTM
standards for thermal properties where appropriate and on the
development of new test standards where necessary It is not
the intent of this guide to rigidly specify particular thermal test
methods for advanced ceramics Guidance is provided on how
to utilize the most commonly available ASTM thermal test
methods, considering their capabilities and limitations
1.5 The values stated in SI units are to be regarded as
standard No other units of measurement are included in this
standard SeeIEEE/ASTM SI 10
1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use It is the
responsibility of the user of this standard to establish
appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.
2 Referenced Documents
2.1 ASTM Standards:2 2.1.1 Specific Heat:
C351Test Method for Mean Specific Heat of Thermal Insulation(Withdrawn 2008)3
D2766Test Method for Specific Heat of Liquids and Solids
E1269Test Method for Determining Specific Heat Capacity
by Differential Scanning Calorimetry
2.1.2 Thermal Conductivity:
C177Test Method for Steady-State Heat Flux Measure-ments and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus
C182Test Method for Thermal Conductivity of Insulating Firebrick
C201Test Method for Thermal Conductivity of Refractories
C202Test Method for Thermal Conductivity of Refractory Brick
C408Test Method for Thermal Conductivity of Whiteware Ceramics
C518Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
C767Test Method for Thermal Conductivity of Carbon Refractories
C1044Practice for Using a Guarded-Hot-Plate Apparatus or Thin-Heater Apparatus in the Single-Sided Mode
C1045Practice for Calculating Thermal Transmission Prop-erties Under Steady-State Conditions
C1113Test Method for Thermal Conductivity of Refracto-ries by Hot Wire (Platinum Resistance Thermometer Technique)
C1114Test Method for Steady-State Thermal Transmission Properties by Means of the Thin-Heater Apparatus
C1130Practice for Calibrating Thin Heat Flux Transducers
E1225Test Method for Thermal Conductivity of Solids
1 This guide is under the jurisdiction of ASTM Committee C28 on Advanced
Ceramics and is the direct responsibility of Subcommittee C28.03 on Physical
Properties and Non-Destructive Evaluation.
Current edition approved Feb 1, 2013 Published March 2013 Originally
approved in 2000 Last previous edition approved in 2006 as C1470 – 06 DOI:
10.1520/C1470-06R13.
2 For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org For Annual Book of ASTM Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
3 The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 United States
Trang 2Using the Guarded-Comparative-Longitudinal Heat Flow
Technique
E1530Test Method for Evaluating the Resistance to
Ther-mal Transmission of Materials by the Guarded Heat Flow
Meter Technique
2.1.3 Thermal Expansion:
C372Test Method for Linear Thermal Expansion of
Porce-lain Enamel and Glaze Frits and Fired Ceramic Whiteware
Products by the Dilatometer Method
C1300Test Method for Linear Thermal Expansion of Glaze
Frits and Ceramic Whiteware Materials by the
Interfero-metric Method
E228Test Method for Linear Thermal Expansion of Solid
Materials With a Push-Rod Dilatometer
E289Test Method for Linear Thermal Expansion of Rigid
Solids with Interferometry
E831Test Method for Linear Thermal Expansion of Solid
Materials by Thermomechanical Analysis
2.1.4 Thermal Diffusivity:
C714Test Method for Thermal Diffusivity of Carbon and
Graphite by Thermal Pulse Method
E1461Test Method for Thermal Diffusivity by the Flash
Method
2.1.5 Emittance/Emissivity:
E408Test Methods for Total Normal Emittance of Surfaces
Using Inspection-Meter Techniques
E423Test Method for Normal Spectral Emittance at
El-evated Temperatures of Nonconducting Specimens
2.1.6 General Standards:
C373Test Method for Water Absorption, Bulk Density,
Apparent Porosity, and Apparent Specific Gravity of Fired
Whiteware Products, Ceramic Tiles, and Glass Tiles
C1145Terminology of Advanced Ceramics
E122Practice for Calculating Sample Size to Estimate, With
Specified Precision, the Average for a Characteristic of a
Lot or Process
E473Terminology Relating to Thermal Analysis and
Rhe-ology
E1142Terminology Relating to Thermophysical Properties
C1045Practice for Calculating Thermal Transmission
Prop-erties Under Steady-State Conditions
IEEE/ASTM SI 10Standard for Use of the International
System of Units (SI) (The Modern Metric System)
3 Terminology
3.1 Definitions:
3.1.1 advanced ceramic, n—a highly engineered,
high-performance, predominantly nonmetallic, inorganic, ceramic
material having specific functional attributes ( C1145 )
3.1.2 ceramic matrix composite, n—a material consisting of
two or more materials (insoluble in one another), in which the
major continuous component (matrix component) is a ceramic,
while the secondary component/s (reinforcing component) may
be ceramic, glass-ceramic, glass, metal, or organic in nature
These components are combined on a macroscale to form a
useful engineering material possessing certain properties or
behavior not possessed by the individual constituents ( C1145 )
3.1.3 coeffıcient of linear thermal expansion, α[T-1], n—the
change in length, relative to the length of the specimen, accompanying a unit change of temperature, at a specified temperature [This property can also be considered the instan-taneous expansion coefficient or the slope of the tangent to the
∆L/L versus T curve at a given temperature.] ( E1142 )
3.1.4 continuous fiber-reinforced ceramic composite
(CFCC), n—a ceramic matrix composite in which the
reinforc-ing phase(s) consists of continuous filaments, fibers, yarns, or
3.1.5 differential scanning calorimetry (DSC), n—a
tech-nique in which the difference in energy inputs into a test specimen and a reference material is measured as a function of temperature while the test specimen and reference material are subjected to a controlled temperature program (E1269)
3.1.6 discontinuous fiber-reinforced ceramic composite,
n—a ceramic matrix composite reinforced by chopped fibers.
( C1145 )
3.1.7 emittance (emissivity), ε (nd), n—the ratio of the
radiant flux emitted by a specimen per unit area to the radiant flux emitted by a black body radiator at the same temperature and under the same conditions Emittance ranges from 0 to 1, with a blackbody having an emittance of 1.00 ( E423 )
3.1.8 linear thermal expansion, [nd], n—the change in
length per unit length resulting from a temperature change
Linear thermal expansion is symbolically represented by ∆L/
L0, where ∆L is the observed change in length ∆L = L2– L1,
and L0, L1, and L2are the lengths of the specimen at reference
temperature T0and test temperatures T1and T2 ( E228 )
3.1.9 mean coeffıcient of linear thermal expansion, α L [T-1],
n—the change in length, relative to the length of the specimen,
accompanying a unit change of temperature measured across a
specified temperature range (T1to T2) ( C372 )
3.1.10 particulate reinforced ceramic matrix composite,
n—a ceramic matrix composite reinforced by ceramic
3.1.11 specific heat (specific heat capacity), C [mL–1T–2θ–1],
n—the quantity of heat required to provide a unit temperature
3.1.12 thermal conductivity, λ [mLT–1θ–1], n—the time rate
of heat flow, under steady conditions, through unit area, per unit temperature gradient in the direction perpendicular to the
3.1.13 thermal diffusivity, [L2T–1], n—the property given by
the thermal conductivity divided by the product of the bulk
3.1.14 thermodilatometry, n—a technique in which a
dimen-sion of a test specimen under negligible applied force is measured as a function of temperature while the test specimen
is subjected to a controlled temperature program in a specified
3.2 Units for Thermal Properties:
Trang 3Property SI Units Abbreviation
Specific heat capacity joules/(gram-kelvin) J/(g·K)
Thermal Conductivity watts/(metre-kelvin) W/(m·K)
Thermal diffusivity metre/second 2
m/s 2
Coefficient of Thermal
Expansion
metre/(metre-kelvin) K –1
Emittance/emissivity no dimensions —
4 Summary of Guide
4.1 Five thermal properties (specific heat capacity, thermal
conductivity, thermal diffusivity, thermal expansion, and
emittance/emissivity) are presented in terms of their definitions
and general test methods The relationship between thermal
properties and the composition, microstructure, and processing
of advanced ceramics is briefly outlined, providing guidance
on which material characteristics have to be considered in
evaluating the thermal properties Additional sections describe
sampling considerations, test specimen preparation, and
report-ing requirements
4.2 Current ASTM test methods for thermal properties are
tabulated in terms of test method concept, testing range,
specimen requirements, standards/reference materials,
capabilities, limitations, precision, and special instructions for
monoliths and composites
5 Significance and Use
5.1 The high-temperature capabilities of advanced ceramics
are a key performance benefit for many demanding engineering
applications In many of those applications, advanced ceramics
will have to perform across a broad temperature range The
thermal expansion, thermal diffusivity/conductivity, specific
heat, and emittance/emissivity are crucial engineering factors
in integrating ceramic components into aerospace, automotive,
and industrial systems
5.2 This guide is intended to serve as a reference and
information source for testing the thermal properties of
ad-vanced ceramics, based on an understanding of the
relation-ships between the composition and microstructure of these
materials and their thermal properties
5.3 The use of this guide assists the testing community in
correctly applying the ASTM thermal test methods to advanced
ceramics to ensure that the thermal test results are properly
measured, interpreted, and understood This guide also assists
the user in selecting the appropriate thermal test method to
evaluate the particular thermal properties of the advanced
ceramic of interest
5.4 The thermal properties of advanced ceramics are critical
data in the development of ceramic components for aerospace,
automotive, and industrial applications In addition, the effect
of environmental exposure on thermal properties of the
ad-vanced ceramics must also be assessed
6 Procedure
6.1 Review Sections7 – 10to become familiar with thermal
property concepts and thermal testing issues for advanced
ceramics, specimen preparation guidance, and reporting
rec-ommendations
6.2 Review the test method text and tables in Section11for
the property you need to determine Use the text and tables to
help select the most appropriate ASTM test method for evaluating the thermal property of interest for the specific advanced ceramic
6.3 Perform the thermal property test in accordance with the selected ASTM test method, but refer back to the guide for directions and recommendations on material characterization, sampling procedures, test specimen preparation, and reporting results
7 Thermal Properties and Their Measurement
7.1 Specific Heat Capacity:
7.1.1 Specific heat capacity is the amount of energy required
to increase the temperature by one unit for a unit mass of material It is a fundamental thermal property for engineers and scientists in determining the temperature response of materials
to changes in heat flux and thermal conditions The SI units for specific heat capacity are joules/(gram·K) Since the specific heat capacity changes with temperature, a specific heat capac-ity value must always be associated with a specific test temperature or temperature range
7.1.2 Specific heat capacity is commonly measured by calorimetry in which changes in thermal energy are measured against changes in temperature The two common calorimetry methods are differential scanning calorimetry and drop calo-rimetry
7.1.3 Differential scanning calorimetry heats the test mate-rial at a controlled rate in a controlled atmosphere through the temperature region of interest The heat flow into the test material is compared to the heat flow into a reference material
to determine the energy changes in the test material as a function of temperature
7.1.4 In drop calorimetry, the test sample is heated to the desired temperature and then immersed in an instrumented, liquid-filled container (calorimeter), which reaches thermal equilibrium The increase in temperature of the calorimeter liquid/container is a measure of the amount of heat in the test specimen
7.1.5 In any calorimetry test, the experimenter must recog-nize that phase changes and other thermo-physical transforma-tions in the material will produce exothermic and endothermic events which will be captured in the test data The thermal events must be properly identified and understood within the context of the material properties, chemistry, and phase com-position across the temperature range of interest
7.2 Thermal Conductivity:
7.2.1 Thermal conductivity is a measurement of the rate of heat flow through a material for a given temperature gradient
It is normalized for thickness and cross-sectional area to give
a material specific value The thermal conductivity of a ceramic
is used in determining the effectiveness of a ceramic either as
a thermal insulator or as a thermal conductor The SI units for thermal conductivity are watts/(metre·kelvin) As with other thermal properties, thermal conductivity changes with temperature, so that a thermal conductivity value for a material must be associated with a specific test temperature
7.2.2 In electrically nonconductive ceramics, thermal con-ductivity occurs by lattice vibration (phonon) concon-ductivity and
Trang 4by radiation (photon) at higher temperatures (>500°C)
Ther-mal conductivity decreases when the mean free path of the
phonons and photons decreases Lattice imperfections,
differ-ences in atomic weight between anions and cations,
non-stoichiometric compositions, solid solutions, amorphous
atomic structures, porosity, and grain boundaries all act as
scattering sites for phonons and reduce the thermal
conductiv-ity of the material
7.2.3 Thermal conductivity is commonly measured by
steady-state methods, that is, cut bar comparative techniques,
heat flow meter techniques, guarded hot-plate/heater/hot-wire
techniques, and calorimetry techniques It can also be
deter-mined by transient techniques (hot wire and flash diffusivity)
7.2.4 In cut bar comparative heat flow techniques, the test
specimen is subjected to a known heat flow and the
tempera-ture differential/s are measured across the dimension/s of
interest The entire test system with the test specimen is
configured with insulation and heaters to minimize heat flow
perpendicular to the direction of interest In the heat flowmeter
technique, the heat flow in the test is measured by a calibrated
heat flux transducer without the use of direct reference
mate-rials in the test system In both techniques, the thermal
conductivity of the material is then calculated as follows:
where:
λ = thermal conductivity,
q = heat flow/unit area,
∆L = distance across which the temperature difference is
measured, and
∆T = measured temperature difference
Thermal conductivity can be calculated from thermal
diffu-sivity measurements, using the specific heat capacity and the
material density as follows:
λ 5 thermal diffusivity*specific heat capacity*density (2)
7.3 Thermal Expansion (Thermodilatometry):
7.3.1 All materials expand or contract with changes in
temperature, with most materials expanding with increasing
temperature Thermal expansion is often very important in
engineering applications, because differences in thermal
ex-pansion between fitted or bonded components can produce
thermal stresses, leading to component failure
7.3.2 The thermal expansion for a given ceramic
composi-tion and phase is a funccomposi-tion of crystal structure and of atomic
bond strength In ceramics with anisotropic crystal structures,
the thermal expansion is different along the different crystal
axes (This is of particular concern for single crystal specimens
and for specimens with oriented grain structures) For some
specific ceramics (cordierite, aluminum titanate, and zirconium
phosphate), the thermal expansion may be zero or negative in
certain crystal axes in specific temperature regimes
7.3.3 The quantitative determination of the change in
di-mension as a function of temperature is defined as the mean
coefficient of linear thermal expansion – the ratio of a given
change in length per unit length for a specimen for a specific
change in temperature as follows:
α 5@~L2 2 L1!/~T2 2 T1!#/L1 (3)
where:
L1and L2 = lengths of the test specimen at test temperatures
T1and T2, respectively, where T2> T1 7.3.4 The units for mean coefficient of linear thermal expansion are metres/(metre · K) The mean coefficient of linear thermal expansion for a material has to be defined for a given temperature range, for example, 7 × 10–6m/(m·K) for 25
to 500°C
7.3.5 The mean coefficient of linear thermal expansion across a temperature range is different than the instantaneous coefficient of thermal expansion, which is the tangent slope at
a specific temperature for the expansion-temperature curve of the sample
7.3.6 Thermal expansion is commonly measured by thermodilatometry, a technique in which a known dimension of
a test specimen under negligible applied force is measured as
a function of temperature while the specimen is subjected to a controlled-temperature program in a specified atmosphere The measurement of the dimensional change can be done by direct mechanical measurement or by optical techniques (interferom-etry and optical lever)
7.3.7 Different crystalline phases in ceramics have different thermal expansion characteristics Major phase changes in ceramics can give abrupt or progressive changes in length with increasing or decreasing temperature and may cause confusion when included in overall expansion measurements In a similar manner, crystallization and changes in amorphous, glassy phases in ceramics can produce marked changes in the thermal expansion over specific temperature regimes
7.3.8 Thermal expansion measurements for ceramics often show heating/cooling hysteresis when microcracking or minor phase changes occur There are also irreversible thermal expansion effects, based on annealing, heating rate, creep, crystallization, or microcracking Thermal expansion tests done only on heating can be misleading, and thermal expansion measurements should be done under both heating and cooling conditions
7.4 Thermal Diffusivity:
7.4.1 Thermal diffusivity is a measurement of the rate of temperature change in a material measured under transient conditions It is defined as the ratio of the thermal conductivity
to the “specific heat capacity per unit volume” (which is the specific heat capacity divided by the bulk density), as follows:
Diffusivity 5 λ/~Cp/ρ! (4) where:
λ = thermal conductivity,
C p = specific heat capacity, and
ρ = the bulk density,
all at the specific temperature of measurement The units of thermal diffusivity are metre/second2 7.4.2 Thermal diffusivity is important in characterizing the transient thermal response of ceramics that are used in heat transfer applications, either as insulators or as thermal conduc-tion paths As with the other thermal properties that change with temperature, a specific thermal diffusivity value must be defined for a specific temperature or a temperature range
Trang 57.4.3 Thermal diffusivity is experimentally determined by
measuring the temperature-time response of a material to a
thermal event The current method of choice is flash diffusivity,
in which one side of a specimen of known thickness and
temperature is subjected to a short duration thermal pulse The
energy of the pulse is absorbed on the front surface of the
specimen and the resulting rear face temperature rise is
measured Thermal diffusivity is calculated from the specimen
thickness and the time required for the rear face temperature to
reach a specified percentage of its maximum value
7.4.4 All of the microstructural variables that have an
impact on thermal conductivity will have a similar effect on the
thermal diffusivity
7.5 Emittance/Emissivity:
7.5.1 All objects radiate energy depending on their
tempera-ture and their radiative characteristics This radiation is called
thermal radiation, because it is strongly dependent on
tempera-ture A perfect emitter (a blackbody) emits radiation across a
range of wavelengths according to its surface temperature
through the Planck radiation equation The amount of
hemi-spherical radiated energy per unit surface area at a given
temperature for a blackbody is calculated through the
Stefan-Boltzmann’s law as follows:
E b,total 5 5.670 3 10 28*T4@W/~m 2 ·K 4!# (5)
where:
T = specimen temperature for hemispherical emittance
7.5.2 At high temperatures, the emittance of radiation by a
material has significant impact on its thermal condition, based
on how the thermal radiation is emitted, absorbed, and
re-flected For advanced ceramics operating at high temperatures
(>600°C) where radiation is a major mode of heat transfer, the
emittance properties of the ceramic are necessary to model the
thermal conditions and to determine the heat transfer rates
under heating and cooling conditions
7.5.3 The measurement of emittance has a directional
com-ponent based on the observation angle to the plane of the
sample Directional emittance is measured at a specific
obser-vation angle Normal emittance is a special case of directional
emittance, measured normal to the plane of the sample
Hemispherical emittance is measured by integration over the
entire range of solid observation angles
7.5.4 Emittance can also be characterized as either “total” or
“spectral.” Total emittance is a measurement of the radiant
energy across the entire range of thermal wavelengths and is
commonly utilized for total radiation pyrometry and radiant heat transfer analysis Spectral emittance is a measurement of the radiant energy at/across a particular portion of the wavelength/frequency spectrum The emittance at a particular frequency is important in temperature measuring equipment, such as optical pyrometers
7.5.5 Most materials are not perfect emitters The emission
of radiation from a surface depends on many factors:
(impurities, coatings, and oxidation), optical transparency, surface profile/roughness, radiation wavelength, and observa-tion angle The relative radiant flux of a given material can be characterized by the term emittance (also called emissivity) Emittance is the ratio of the radiant flux emitted by a specimen per unit area to the radiant flux emitted by blackbody radiator
at the same temperature and under the same conditions 7.5.6 Emittance is determined by measuring the emitted thermal radiation at a given temperature and then comparing it
to a reference standard of known emittance or by direct comparison to an experimental “blackbody” at the same temperature
8 Test Specimen Characterization
8.1 Introduction:
8.1.1 Advanced ceramics, both monolithic and composite, offer a wide range of thermal properties, from thermal insula-tors to thermal conducinsula-tors Nominal thermal property values for a range of advanced monolithic ceramics are given inTable
1 Note the range of thermal properties listed in the table Advanced ceramics such as aluminum nitride and beryllia with their high thermal conductivity are often used as thermal conductors
8.1.2 Table 2 provides nominal thermal properties for dif-ferent types of ceramic fibers used in continuous fiber-reinforced ceramic matrix composites When these different ceramic fibers are combined with different ceramic matrices, the resulting composites can contain constituents whose ther-mal properties are widely different
8.1.3 The range of thermal properties for advanced ceramics and the complexity inherent in ceramic composites require a detailed understanding of the relationships between composition, processing, microstructure, and thermal proper-ties in those ceramics With that understanding, test operators will ensure that thermal test results for advanced ceramics are valid, useful, and reproducible
TABLE 1 Nominal Thermal Properties for Monolithic Ceramics at Room Temperature
N OTE 1—Thermal property data obtained from reference books and producer specifications Values are approximate for a given class of material and are provided for the sake of general comparison Actual values in specific test specimens will depend on composition, microstructure, porosity, and other factors Emittance/emissivity data from—Thermal Radiative Properties, Nonmetallic Solids, Touloukian, Y S and DeWitt, D.P., I.F I Plenum, 1972 Property at Room Temperature Alumina,
99.5 %
Silicon Nitride
Silicon Carbide
Aluminum Nitride
Boron Nitride Zirconia Mullite Beryllia Cordierite
Thermal diffusivity, m 2
/s × 10 -6
Coefficient of linear thermal
expansion, 10 -6 /K
8.0 (25–1000°C)
3.0 (25–1000°C)
4,4 (25–1000°C)
5.7 (25–1000°C)
10 (25–1000°C)
10.3 (25–1000°C)
5.3 (25–1000°C)
8.7 (25–1000°C)
1.7 (25–1000°C) Emittance/Emissivity, 1000°C 0.1-0.3 0.7-0.9 0.7-0.9 0.7-0.9 0.7-0.9 0.1-0.3 0.4 '0.3 NA
Trang 68.2 Material Characteristics and Thermal Properties:
8.2.1 Advanced ceramics cover a broad range of
compositions, microstructures, and physical properties It is not
possible to give specific guidance for every current or future
advanced ceramic material, but general guidelines and
infor-mation are provided on the effects of composition,
microstructure, and processing on the thermal properties of
advanced ceramics and on thermal property measurement
8.2.2 The thermal properties of an advanced ceramic
(monolithic or composite) are a function of the material
composition and constituents, its microstructure, its processing
history, and its environmental exposure For example, porosity
has a very strong effect on thermal conductivity and diffusivity
In a similar manner, composites containing high thermal
conductivity components can be tailored to specific thermal
property targets by changing the amount and the architecture of
the reinforcing component The advanced ceramic must be
adequately characterized in terms of composition, constituents,
microstructure, processing methods, and exposure history
8.2.3 The sensitivity of the thermal properties of ceramics to
such variations requires that the composition and constituents
in advanced ceramics be sufficiently determined and
docu-mented to avoid misinterpretation of results and to permit
adequate characterization of the test material This
character-ization may be simple and straightforward for monolithic
ceramics, but may require extensive microstructural, chemical,
and physical analysis for complex systems such as whisker or
fiber-reinforced composites The degree of characterization
required depends on the variation in the thermal properties
produced by changes in composition, constituents, and
micro-structure Spatial variations must be considered and adequately
evaluated, particularly for whisker- and particle-reinforced
composites, in which reinforcement concentrations may vary
spatially through the test specimen
8.2.4 For advanced ceramic composites (whisker,
chopped-fiber, and continuous-fiber reinforced) and for monolithics with
oriented or textured grain growth, anisotropy effects must be
carefully evaluated and characterized Directional effects are
pronounced in thermal conductivity/diffusivity and thermal
expansion measurements The character and degree of
anisot-ropy in such composites must be adequately characterized and
understood
8.2.5 In a similar manner, the exposure history of a compo-nent can also affect the thermal properties through oxidation, phase changes, grain growth, high-temperature reactions, corrosion, and slow crack growth Exposure history (time, temperature, and atmosphere) for test specimens has to be well-documented
8.2.6 Exposure and thermal effects can also occur during the thermal testing, changing the composition and microstructure
It is imperative that the test operator be aware of potential reactions, oxidation, phase changes, and other thermal events that could occur across the test temperature range This can occur if the test temperature exceeds the maximum processing temperature or if oxidation reactions (surface or bulk) occur at elevated temperatures Oxidation-sensitive materials should be tested in inert atmospheres Accelerated heating rates can also produce test anomalies, because of nonuniform temperatures within a test specimen Thermal tests should always be done with the test specimen in relative steady-state thermal equilibrium, unless transient properties are specifically desired
8.3 Monolithic Ceramics—Material Variables:
8.3.1 For monolithic ceramics, the following material char-acteristics should be carefully considered in terms of their expected and actual effect on the thermal properties Evalua-tion of the material characteristics may be recommended, if the thermal properties are sufficiently impacted by the pertinent characteristics
8.3.2 Porosity has a major effect on thermal conductivity/ diffusivity From one viewpoint, porosity can be considered an additional, low thermal conductivity/capacity phase in the test specimen in which the pore volume fraction, the porosity size (mean and distribution), and its geometric distribution can all have variable effects on the thermal properties, especially the thermal transport properties The porosity in the test specimens should be adequately defined and characterized with regard to its effect on thermal conductivity/diffusivity
8.3.3 Variations in stoichiometry, impurities, grain size, and grain boundary phases can have a large effect on the thermal conductivity and thermal diffusivity for ceramics with high intrinsic thermal conductivity (aluminum nitride, beryllia, silicon carbide, and so forth) The effect of such composition
TABLE 2 Nominal Thermal Properties of Ceramic Fibers at Room Temperature
N OTE 1—Thermal property data obtained from reference books and producer specifications Values are approximate and are provided for the sake of general comparison Actual values in specific test specimens will depend on composition, microstructure, porosity, architecture, and other factors Property at Room Temperature Nicalon CGA
Hi-NicalonA
Hi-Nicalon SA
Sylramic SiCA
Nextel 312B
Nextel 610B
Nextel 720B T-300
CarbonC
P-120 CarbonC
Nominal composition Silicon
Carbide
Silicon Carbide
Silicon Carbide
Silicon Carbide
Alumino-borosilicate
Alumina
Alumina-Mullite
Carbon Graphite
Thermal conductivity, [W/(m·K)] 2.97 7.77 18.4 40-45 '3(est) '30 (est) '10 (est) 8.5 640 Coefficient of Linear Thermal
Expansion, 10 –6 /K
4.0 (0–900°C)
3.5 (0–500°C)
(20–1320°C)
3.0 (100–1100°C)
7.9 (100–1100°C)
6.0 (100–1100°C)
–1.4 at (23°C)
–1.45 at (23°C)
ACDI Ceramics, Inc., San Diego, CA.
B3M Corp., St Paul, MN.
C
Amoco Performance Products, Alpharetta, GA.
Trang 7variations and phase distributions should be carefully
consid-ered and evaluated (if feasible) in thermal tests of high thermal
conductivity ceramics
8.3.4 Phase changes and devitrification of amorphous
phases may occur during thermal tests, producing anomalies in
the various thermal properties Test operators should be
in-formed of the potential for such transformations in specimens
submitted for testing
8.3.5 Thermal expansion measurements for ceramics often
show heating/cooling hysteresis when microcracking or minor
phase changes occur Irreversible thermal expansion effects
may also occur in certain materials, based on annealing, rapid
heating rates, creep, crystallization, or microcracking Thermal
expansion tests done only on heating can be misleading, and
thermal expansion measurements should be done under both
heating and cooling conditions
8.3.6 Oriented or textured grain structures in monolithic
ceramics can introduce anisotropic effects in the thermal
properties Such grain structures are commonly developed
during processing If the thermal effects of such anisotropy are
significant, the grain structure should be characterized by
optical or SEM microscopy or by X-ray diffraction
Composite—Material Variables:
8.4.1 Particulate and whisker reinforcement of advanced
ceramics adds an additional degree of complexity to the
composition and microstructure of the test specimen It is
critical that the particulate or whisker additions be adequately
characterized in terms of composition, phase and crystal
structure, particle/whisker morphology, and size distribution It
is also useful to have nominal thermal properties, that is,
specific heat, thermal conductivity, and thermal expansion, for
the reinforcement across the temperature range of interest
Those thermal property data will assist in interpreting the
composite thermal test results
8.4.2 As part of the composite specimen, the particulate/
whisker reinforcement must be adequately defined in terms of
the volume fraction, spatial distribution, and orientation/
anisotropy/texture For example, reinforcements with high
thermal conductivity relative to the matrix will have markedly
different effects on the composite thermal properties,
depend-ing on how the reinforcements are distributed and oriented For
example, whiskers in a laminated, planar orientation will
produce anisotropic thermal conductivity in the composite The
bulk thermal conductivity will markedly change if the
particu-late packing factor is high enough to produce significant
particle-to-particle contact
8.4.3 If there is a large difference in thermal expansion
between the reinforcement and the matrix in the composite,
residual stresses or microcracks, or both, can develop during
processing Such residual stresses and microcracks can
pro-duce anomalies in thermal expansion measurements
Microc-racking can also have a direct effect on thermal conductivity/
diffusivity
8.5 Continuous Fiber-Reinforced Ceramic Composites—
Material Variables:
8.5.1 The use of continuous fiber reinforcement in ceramic
composites provides the ceramic engineer the greatest range of
property control and tailoring, but also introduces the highest level of complexity into the composition and microstructure of advanced ceramics
8.5.2 It is essential that the composition and architecture of the fiber reinforcement in the composite be adequately char-acterized and documented, to include the following:
8.5.2.1 Fiber composition, filament morphology (diameter and length), and fiber volume fraction
8.5.2.2 Filament counts in tows, comprehensive description
of the reinforcement architecture [one-dimensional (tows), two-dimensional (woven fabrics), and three-dimensional (weaves and braid)] to include tow count and repeat units 8.5.2.3 The composition and morphology of any interface coatings on the fibers, used for process protection of the fibers
or for development of crack deflection modes in the composite 8.5.2.4 The composition and morphology (thickness, porosity, and grain structure) of any surface coatings on the composite component, used for surface sealing, oxidation/ corrosion protection, or wear/abrasion resistance Surface coat-ings may be considered as monolithic layers on the surface of the composite
8.5.3 The matrix in the composite must also be adequately characterized in terms of composition, constituents, morphology, and grain structure Porosity in the composite should also be characterized, considering volume fraction, pore size, shape factors, and distribution of porosity
8.5.4 Fiber-reinforced ceramic composites often have strong anisotropic properties, determined by the reinforcement archi-tecture with higher fiber volume loadings in specific directions
It is essential that the geometry and orientation of the rein-forcement are fully documented and correlated with the ther-mal test results
8.5.5 In many thermal tests (such as thermal dilatometery, diffusivity, and specific heat), the test specimens may have one
or more dimensions which are small in comparison to the weave repeat elements Experimenters should carefully con-sider the size of the test specimens for a specific test to ensure that an adequate number of weave repeat elements are included
in the specimen Specimens of insufficient size may not be representative of the properties of the larger piece or may have exaggerated end or edge effects
9 Test Specimen Sampling and Preparation
9.1 Test Specimen Sources—Test specimens for thermal
evaluation can be taken from engineering components or fabricated test panels/billets The selection of a particular source depends on the required test specimen geometry and the suitability of the test component for test specimen preparation The primary objective is to select test specimens that are representative of the composition, processing, and properties of the final functional part
9.2 Specimen Sampling to Assess Variability:
9.2.1 Depending on the degree of process control, variabil-ity may occur among test specimens within a batch and between different batches of test specimens The variation may occur in the composition, microstructure, and porosity of advanced ceramics The degree of variation will depend on the homogeneity within the batch and on reproducibility between
Trang 8batches from the producer The test operator should select/
prepare sufficient test specimens to provide a representative
sample of the entire batch of specimens submitted for testing
In addition, batch/lot identification should be carefully noted
for documentation purposes
9.2.2 Practice E122 provides guidance on calculating the
number of units required for testing to obtain an estimate of
certain precision for a given property
9.3 Orientation and Anisotropy Effects—If the test material
has significant anisotropy or orientation effects, it is imperative
that test specimens be selected to adequately sample the
anisotropy in the major directions of interest A minimum of
two directions should be selected for test specimens The
orientations should be selected to produce the maximum and
minimum material properties Each test specimen should be
marked and identified so that its orientation in the original
part/test piece can be identified
9.4 Location and Identification:
9.4.1 Given the variability which may occur in
developmen-tal advanced ceramics and the anisotropy which may be an
engineered characteristic of composite components, test
speci-men locations should be docuspeci-mented by drawing or
photo-graph Test specimens should be identified so that their location
can be traced on the original part Special identification should
be used for test specimens taken from plate edges or from areas
with local anomalies
9.4.2 Nondestructive evaluation (NDE) methods may be of
value in characterizing test plates prior to specimen
preparation, particularly for fiber-reinforced ceramic
compos-ites The NDE results will assist in locating material anomalies,
such as excessive porosity or delaminations
9.5 Specimen Count and Repeat Tests—Follow the
instruc-tions of the specific test method to determine the specimen
count and repeat tests needed for the required precision and
confidence
9.6 Surface Effects on Thermal Property Measurements—
Surface finish requirements are commonly detailed in thermal
test methods to provide for minimal thermal resistance at
contact surfaces or to ensure flat, smooth, and accurate surfaces
for measurement Surface finish can have a strong effect on
radiation absorption/emission with a direct effect on emittance
measurements and on thermal diffusivity measurements by
flash Surface finish measurements may be advisable to ensure
reproducibility and proper interpretation of specific thermal
test results
9.7 Test Specimen Preparation:
9.7.1 Careful machining of specimens is critical in all
thermal measurements, primarily for accurate measurement of
critical dimensions and for accurate fit of specimens into
fixtures Machining procedures should be sufficiently
docu-mented for reproducibility
9.7.2 Per Test MethodC372, test specimens for mechanical
dilatometry should have ends that are cut/ground flat and
perpendicular to the specimen axis Dilatometer fixtures are
commonly designed to provide point contact with specimens
and to prevent sideways levering For thermal expansion
measurements using optical test methods, specimen ends need
to be flat and parallel to a quite high precision to ensure stability and to define light paths
9.7.3 There is no standard method for machining and finishing ceramic specimens for thermal testing There are three general categories of machining which can be applied both to monolithic and composites ceramics
9.7.3.1 Application-Matched Machining—The machined
surfaces of the thermal test specimen will have the same surface/edge preparation as that given to a service component
9.7.3.2 Customary Practices—In instances where a
custom-ary machining procedure has been developed that is completely satisfactory for a class of materials (that is, it induces no unwanted surface/subsurface damage or residual stresses), this procedure may be used to produce the thermal specimens
9.7.3.3 Recommended Procedure—In instances where
Application-Matched Machining and Customary Practices are not pertinent, the following machining procedures are recom-mended as a method commonly used for ceramic test speci-mens for mechanical testing
(1) Perform all grinding or cutting with ample supply of
appropriate filtered coolant to keep the specimen and grinding wheel constantly flooded and particles flushed Grinding can be done in two stages, ranging from coarse to fine rates of material removal All cutting can be done in one stage appropriate for the depth of cut
(2) The stock removal rate shall not exceed 0.03 mm per
pass to the last 0.06 mm of material removed Final finishing shall use diamond tools between 320 and 500 grit No less than 0.06 mm shall be removed during the final finishing stage, and
at a rate less than 0.002 mm per pass Remove equal stock from opposite faces
(3) Grinding is followed by either annealing or lapping, as
deemed appropriate for a particular material and test For silicon based ceramics (for example, silicon carbide or silicon nitride) or oxide ceramics containing a glassy phase (for example, aluminas with second phases) annealing at ;1200°C for ;2 h is generally sufficient to heal the grinding damage introduced during specimen preparation
9.7.4 Specimens may require environmental or thermal conditioning, prior to testing, if there is a need to determine changes in thermal properties after such environmental or thermal exposure Any conditioning procedures should be fully documented with careful measurements of dimensions, weights, and appearance before and after conditioning 9.7.5 Record dimensions and weights of all samples to the required tolerances or 61 %, whichever is more restrictive Dimensions and weights are important in calculating specimen densities However, bulk density and apparent porosity should
be calculated using Archimedes immersion (Test Method C373) where necessary, especially for composites with signifi-cant porosity or surface roughness
9.8 Post Test Inspection—In any thermal test involving
high-temperature exposure, post-test examination of the tested specimen is of value in determining thermally induced changes, such as melting, sintering, phase changes, oxidation,
or corrosion Visual inspection should be done as a minimum
Trang 9Dimensional inspection or microstructural and compositional
analysis, or both, may be necessary if such thermal changes are
suspected
9.9 Health and Safety—Proper handling and safety
proce-dures should be used for all ceramic materials in monolithic or
composite forms Refer to the Material Safety Data Sheets for
the test materials for inhalation, handling, ingestion, reaction,
fire, environmental, and disposal hazards
10 Report
10.1 Follow all of the normal reporting requirements of the
standard used for testing
10.2 The accurate description of the advanced ceramic test
specimens is essential for proper interpretation of the thermal
test results All ceramic test specimens should be described to
the fullest extent possible, providing documentation (as
applicable/available) on:
10.2.1 Material composition, chemistry, phase, and
microstructure,
10.2.2 Composite composition and architecture,
10.2.3 Processing and fabrication method and exposure
history,
10.2.4 Part description,
10.2.5 Sampling method and identification,
10.2.6 Specimen preparation, conditioning, and geometry,
and
10.2.7 Equipment and test procedure description
11 Test Method Comparison
11.1 Thermal Conductivity Test Methods—There are five
general types of ASTM test standards for thermal conductivity:
Cut Bar Comparative Heat Flow, Guarded Heat Flow Meter,
Guarded-Hot-Plate, Hot Wire, and Calorimetry Method for
Refractories
11.1.1 Cut Bar Comparative Heat Flow Technique—A test
specimen of measured cross section and thickness is
sand-wiched between two identical reference specimens of a
mate-rial with known thermal conductivity The test assembly
(reference specimen/test specimen/reference specimen) is
placed with an applied force between two heating elements
controlled at different temperatures to establish the desired
thermal gradient along the length of the test assembly The test
assembly is designed (with heaters or insulation) to eliminate/
minimize radial/lateral heat losses The temperature gradient is
measured by thermocouples at six known locations in the test
assembly: two locations in the upper reference specimen, two
locations in the test specimen, and two locations in the lower
reference specimen Once thermal equilibrium is reached, the
heat flux in the test assembly is calculated in the two reference
specimens based on the known thermal conductivity and the
temperature gradients in the two references Knowing the heat
flux from the reference specimens and the thermal gradient in
the test specimens, the thermal conductivity of the test
speci-men is calculated (See Test Methods E1225 andC408.) See
Fig 1
11.1.2 Guarded Heat Flow Meter Technique—The heat flow
meter apparatus establishes a steady-state unidirectional heat
flux through a test specimen between two parallel plates at
constant but different temperatures The heat flux is measured
by a heat flux transducer (output voltage changes with heat flow through the sensor) in the instrument, which is calibrated with one or more reference materials of known thermal conductivity At thermal equilibrium the measured heat flux, the temperature drop between the top and bottom faces of the specimen, and the specimen thickness are used to calculate the thermal conductivity of the test specimen (See Test Methods C518 andE1530.) SeeFig 2
11.1.3 Guarded-Hot-Plate Technique—Test specimens of
fixed thickness are placed between a main heat source operat-ing at a known power level and an auxiliary heater operatoperat-ing at
a fixed temperature Additional guard heaters are positioned to minimize lateral heat flow Thermocouples measure the tem-perature differential across the test specimen thickness at thermal equilibrium The hot-plate technique can be used in either a two-sided or a one-sided mode The Thin-Heater Method is a geometrical variation of the guarded hot plate
FIG 1 Schematic of Test Stack and Guard System for Cut Bar
Comparative Heat Flow Technique
FIG 2 Schematic of Guarded Heat Flow Meter Apparatus
Trang 10technique (See Test Method C177, Practice C1044, and Test
MethodC1114.) SeeFig 3
11.1.4 Hot Wire Technique—In the hot-wire technique, a
resistance heated platinum wire is sandwiched between two
blocks of the test material in a temperature controlled chamber
A constant electrical current is applied to the wire The rate at
which the wire heats is dependent on how rapidly heat flows
from the wire into the constant temperature mass of the test
blocks The rate of temperature increase of the wire is
accurately determined by measuring its increase in resistance
in the same way a platinum resistance thermocouple is used A
Fourier equation is used to calculate the thermal conductivity
based on the rate of temperature increase of the hot wire and
the power input (See Test MethodC1113.) SeeFig 4
11.1.5 Calorimetry Technique for Refractories—A
refrac-tory insulation specimen of known thickness is positioned in an
electrically heated furnace with the back side of the specimen
positioned against a water-cooled calorimeter (a copper plate
with flowing water) The inner and outer face of the refractory
specimen are instrumented with thermocouples The refractory
test specimen is fixtured with insulating guard bricks to reduce
lateral heat flow With the furnace at the desired temperature
and the calorimeter showing a steady-state heat flow, the heat
flux flowing through the test specimen is determined from the
measured heat flow and the specimen cross-sectional area The
thermal conductivity is then calculated from the heat flux and
the temperature gradient across the specimen thickness (See
Test MethodsC182,C201,C202, andC767.) SeeFig 5
N OTE 1—The calorimetry technique has been superceded by simpler,
less complex conductivity methods and is not commonly used for
advanced ceramics, because of the large sample size and the mass and the
cost of the test equipment.
11.1.6 General requirements and guidelines for determining
the thermal transmission properties based upon heat flux
measurements are given in PracticeC1045.Tables 3 and 4give
comparative information on the different ASTM test methods
for thermal conductivity
11.2 Specific Heat Capacity Test Methods—Specific heat
capacity is measured in two ways in accordance with ASTM
methods: drop calorimetry and scanning differential
calorim-etry
11.2.1 Drop Calorimetry—A test specimen is heated to a
specified temperature in a furnace and then rapidly inserted
into an adiabatic calorimeter at a known temperature When thermal equilibrium is reached between the test specimen and the calorimeter, the increase in thermal energy in the calorim-eter is measured based on the temperature increase and the specific heat capacity of the calorimeter The specific heat capacity of the sample is then calculated by dividing the measured increase in thermal energy of the calorimeter by the specimen temperature difference (initial specimen temperature – final specimen temperature) and the mass of the test specimen (See Test MethodsD2766andC351.) SeeFig 6
11.2.2 Differential Scanning Calorimetry—The test
speci-men is heated at a controlled rate in a controlled atmosphere through the temperature regime of interest The heat flow into the test specimen is measured with a heat flux transducer and compared to the heat flow into a reference material or blank run concurrently The difference in heat flow is continually moni-tored and recorded as the test specimen is heated The measured difference in heat flow across the temperature range
of interest is used to calculate the specific heat capacity of the test specimen (See Test MethodE1269.) SeeFig 7
11.2.3 Table 5gives comparative information on the differ-ent ASTM test methods for specific heat capacity
11.3 Thermal Diffusivity Test Methods:
11.3.1 Flash Diffusivity—Thermal diffusivity is
experimen-tally determined by measuring the temperature time response
FIG 3 Schematic of Guarded Hot Plate Apparatus
FIG 4 Schematic of Hot Wire Thermal Conductivity Technique
FIG 5 Schematic of Calorimetry Technique